The present invention relates to stereoselective Ziegler-Natta catalyst compositions for use in the polymerization of propylene having improved control over polymerization activity and reactor process continuity through the use of carefully chosen mixtures of selectivity control agents. Ziegler-Natta propylene polymerization catalyst compositions are well known in the art. Typically, these compositions include a transition metal compound, especially a mixed titanium, magnesium and halide containing compound in combination with an internal electron donor (referred to as a procatalyst); a co-catalyst, usually an organoaluminum compound; and a selectivity control agent (SCA). Examples of such Ziegler-Natta catalyst compositions are shown in: U.S. Pat. Nos. 4,107,413; 4,115,319; 4,220,554; 4,294,721; 4,330,649; 4,439,540; 4,442,276; 4,460,701; 4,472,521; 4,540,679; 4,547,476; 4,548,915; 4,562,173; 4,728,705; 4,816,433; 4,829,037; 4,927,797; 4,990,479; 5,028,671; 5,034,361; 5,066,737; 5,066,738; 5,077,357; 5,082,907; 5,106,806; 5,146,028; 5,151,399; 5,153,158; 5,229,342; 5,247,031; 5,247,032 and U.S. Pat. No. 5,432,244.
Catalyst compositions designed primarily for the polymerization of propylene or mixtures of propylene and ethylene generally include a selectivity control agent in order to affect polymer properties, especially tacticity or stereoregularity of the polymer backbone. As one indication of the level of tacticity, especially the isotacticity of polypropylene, the quantity of such polymer that is soluble in xylene or similar liquid that is a non-solvent for the tactic polymer is often used. The quantity of polymer that is soluble in xylene is referred to as xylene solubles content or XS. In addition to tacticity control, molecular weight distribution (MWD), melt flow (MF), and other properties of the resulting polymer are affected by use of a SCA as well. It has also been observed that the activity of the catalyst composition as a function of temperature may be affected by the choice of SCA. Often however, a SCA which gives desirable control over one polymer property, is ineffective or detrimental with respect to additional properties or features. Conversely, an SCA that is effective in combination with one procatalyst may not be effective when used in combination with a different procatalyst.
It is known that the use of certain alkoxy derivatives of aromatic carboxylic acid esters, especially ethyl p-ethoxybenzoate (PEEB), in combination with a Ziegler-Natta procatalyst composition containing a monoester of an aromatic monocarboxylic acid, exemplified by ethyl benzoate, results in an inferior catalyst composition possessing lower overall polymerization activity and polymers having relatively low isotacticities and increased oligomer contents, all of which are generally undesired results.
Disadvantageously however, alkoxysilane SCA's, exemplified by dicyclopentyldimethoxysilane (DCPDMS), methylcyclohexyldimethoxysilane (MCHDMS) and n-propyltrimethoxysilane (NPTMS) when used in combination with ethylbenzoate internal electron donor results in catalyst compositions that are not generally self-extinguishing. That is, these compositions can give polymerization process control problems, especially sheeting and formation of large polymer chunks due to hard to control temperature excursions allowing polymer particles to form agglomerates. Such catalyst compositions are not “self-extinguishing”. Rather, at higher reaction temperatures, they tend to be more active, resulting in difficult to control processes. In addition, under conditions of a reactor upset or a power outage, the normally fluidized reaction bed of a gas phase polymerization reactor may settle to the diffuser plate of the reactor. In that state, continued polymerization may generate excessive temperatures, resulting in fusion of the entire reactor contents into a solid mass which requires opening of the reactor and laborious effort to remove the polymer mass.
Use of mixtures of SCA's in order to adjust polymer properties is known. Examples of prior art disclosures of catalyst compositions making use of mixed SCA's, especially mixtures of silane or alkoxysilane SCA's include: U.S. Pat. Nos. 5,100,981, 5,192,732, 5,414,063, 5,432,244, 5,652,303, 5,844,046, 5,849,654, 5,869,418, 6,066,702, 6,087,459, 6,096,844, 6,111,039, 6,127,303, 6,133,385, 6,147,024, 6,184,328, 6,303,698, 6,337,377, WO 95/21203, WO 99/20663, and WO 99/58585. References generally showing mixtures of silanes with monocarboxylic acid ester internal electron donors or other SCA's include: U.S. Pat. Nos. 5,432,244, 5,414,063, JP61/203,105, and EP-A-490,451.
Despite the advances occasioned by the foregoing disclosures, there remains a need in the art to provide an aromatic monocarboxylic acid ester internal electron donor containing Ziegler-Natta catalyst composition for the polymerization of olefins, especially propylene and propylene containing mixtures, wherein the catalyst composition retains the advantages of alkoxysilane SCA containing catalyst compositions with regard to polymer properties, but additionally possesses improved temperature/activity properties. Especially desired are such compositions that are inherently self-extinguishing with regard to catalyst activity as a function of temperature, thereby leading to reduced polymer agglomerate formation, improved polymerization process control, and increased immunity to reactor upset or power outages.